In order to detect and study the mechanisms by which some environmental agents induce developmental disorders, an in vitro culture system of rat embryos during the organogenesis phase of development (paralleling in vivo development) has been developed. In this culture system, rat conceptuses of pregnancy days 10, 11 or 12 have been grown for two days. These embryos grow in vitro 5-20 fold with extensive differentiation of major organs, e.g., brain, neural tube, sensory organs, circulatory system, gut, liver, musculature etc., comparable with those in vivo during the same period. Treatment with teratogens, TEM, BUdR, and heparin have been shown to induce different types of developmental anomalies in these embryos grown in vitro.